Perhaps it is ritual for a Vulcan's body to be returned to Vulcan, if it can be. And in this case, Spock COULD have been brought back to Vulca, but Kirk left him on Genesis.

They transfered the katra back to Spock, because they now had his body, so why not, and because he was now LIVING, but with no memories of before. It made sense to give back to Spock what was his, and what is his very essence.

But, no. If they were only bringing the katra to Seleya, then a body was not needed.